How to Do a Systematic Review of Literature
J R Soc Med. 2003 Mar; 96(iii): 118–121.
5 steps to conducting a systematic review
Regina Kunz
1 German Cochrane Center, Freiburg and Department of Nephrology, Charité, Berlin, Germany
Jos Kleijnen
two Centre for Reviews and Dissemination, York, United kingdom of great britain and northern ireland
Gerd Antes
3 German Cochrane Centre, Freiburg, Germany
Systematic reviews and meta-analyses are a central chemical element of testify-based healthcare, even so they remain in some ways mysterious. Why did the authors select certain studies and reject others? What did they practice to pool results? How did a bunch of insignificant findings suddenly become significant? This paper, along with a book1 that goes into more detail, demystifies these and other related intrigues.
A review earns the describing word systematic if it is based on a conspicuously formulated question, identifies relevant studies, appraises their quality and summarizes the prove by utilize of explicit methodology. It is the explicit and systematic approach that distinguishes systematic reviews from traditional reviews and commentaries. Whenever we use the term review in this newspaper it volition mean a systematic review. Reviews should never exist done in any other manner.
In this paper nosotros provide a step-by-step explanation—there are only 5 steps—of the methods behind reviewing, and the quality elements inherent in each stride (Box 1). For purposes of illustration we use a published review concerning the safety of public water fluoridation, merely we must emphasize that our subject is review methodology, non fluoridation.
EXAMPLE: SAFETY OF PUBLIC WATER FLUORIDATION
You are a public wellness professional in a locality that has public h2o fluoridation. For many years, your colleagues and you accept believed that it improves dental wellness. Recently there has been pressure from various interest groups to consider the safety of this public health intervention because they fearfulness that it is causing cancer. Public health decisions have been based on professional judgment and applied feasibility without explicit consideration of the scientific evidence. (This was yesterday; today the evidence is available in a York review2 , 3, identifiable on MEDLINE through the freely attainable PubMed clinical queries interface [http://www.ncbi.nlm.nib.gov/entrez/query/static/clinical.html], under 'systematic reviews'.)
STEP 1: FRAMING THE QUESTION
The research question may initially exist stated equally a query in free form merely reviewers prefer to pose it in a structured and explicit way. The relations between various components of the question and the construction of the research design are shown in Figure 1. This newspaper focuses only on the question of safety related to the outcomes described beneath.
Structured questions for systematic reviews and relations between question components in a comparative study
Free-form question
Is it safe to provide population-wide drinking water fluoridation to prevent caries?
Structured question
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The populations—Populations receiving drinking water sourced through a public water supply
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The interventions or exposures—Fluoridation of drinking h2o (natural or bogus) compared with non-fluoridated water
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The outcomes—Cancer is the main outcome of interest for the argue in your health authority
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The study designs—Comparative studies of whatever blueprint examining the harmful outcomes in at least two population groups, one with fluoridated drinking water and the other without. Harmful outcomes can be rare and they may develop over a long time. At that place are considerable difficulties in designing and conducting safe studies to capture these outcomes, since a large number of people need to be observed over a long flow. These circumstances demand observational, not randomized studies. With this groundwork, systematic reviews on safety have to include evidence from studies with a range of designs.
STEP 2: IDENTIFYING RELEVANT PUBLICATIONS
To capture as many relevant citations as possible, a wide range of medical, environmental and scientific databases were searched to identify principal studies of the effects of water fluoridation. The electronic searches were supplemented by hand searching of Index Medicus and Excerpta Medica back to 1945. Furthermore, various net engines were searched for web pages that might provide references. This endeavour resulted in 3246 citations from which relevant studies were selected for the review. Their potential relevance was examined, and 2511 citations were excluded as irrelevant. The full papers of the remaining 735 citations were assessed to select those primary studies in human being that direct related to fluoride in drinking water supplies, comparing at least two groups. These criteria excluded 481 studies and left 254 in the review. They came from thirty countries, published in fourteen languages between 1939 and 2000. Of these studies 175 were relevant to the question of condom, of which 26 used cancer as an consequence.
Pace 3: ASSESSING Written report QUALITY
Design threshold for report pick
Adequate study blueprint equally a marker of quality, is listed equally an inclusion criterion in Box i. This approach is most applicable when the master source of evidence is randomized studies. Withal, randomized studies are most impossible to conduct at community level for a public health intervention such as water fluoridation. Thus, systematic reviews assessing the safety of such interventions take to include evidence from a broader range of written report designs. Consideration of the type and amount of research likely to exist available led to inclusion of comparative studies of any blueprint. In this fashion, selected studies provided information about the harmful effects of exposure to fluoridated h2o compared with non-exposure.
Quality assessment of safety studies
Later on studies of an acceptable blueprint have been selected, their in-depth assessment for the risk of various biases allows u.s. to estimate the quality of the prove in a more refined manner. Biases either exaggerate or underestimate the 'truthful' event of an exposure. The objective of the included studies was to compare groups exposed to fluoridated drinking water and those without such exposure for rates of undesirable outcomes, without bias. Safety studies should define exposures and outcomes in such a manner that the risk of misclassification is minimized. The exposure is likely to be more accurately ascertained if the report was prospective rather than retrospective and if it was started soon after water fluoridation rather than later. The outcomes of those developing cancer (and remaining free of cancer) are likely to be more than accurately ascertained if the follow-upwards was long and if the cess was blind to exposure status.
When examining how the effect of exposure on upshot was established, reviewers assessed whether the comparing groups were similar in all respects other than their exposure to fluoridated h2o. This is because the other differences may exist related to the outcomes of interest contained of the drinking-h2o fluoridation, and this would bias the comparison. For example, if the people exposed to fluoridated water had other gamble factors that fabricated them more decumbent to accept cancer, the apparent clan betwixt exposure and outcome might exist explained by the more frequent occurrence of these factors among the exposed grouping. The technical discussion for such defects is misreckoning. In a randomized written report, misreckoning factors are expected to be roughly equally distributed between groups. In observational studies their distribution may be unequal. Master researchers tin can statistically adjust for these differences, when estimating the consequence of exposure on outcomes, past use of multivariable modelling.
Put simply, use of a prospective design, robust ascertainment of exposure and outcomes, and control for misreckoning are the generic issues 1 would look for in quality assessment of studies on rubber. Consequently, studies may range from satisfactorily coming together quality criteria, to having some deficiencies, to not meeting the criteria at all, and they can be assigned to one of three prespecified quality categories as shown in Table ane. A quality bureaucracy can and so be developed, based on the degree to which studies comply with the criteria. None of the studies on cancer were in the high-quality category, but this was considering randomized studies were non-existent and command for confounding was not always ideal in the observational studies. At that place were viii studies of moderate quality and 18 of low quality.
Table 1
Description of quality assessment of studies on prophylactic of public h2o fluoridation
| Quality categories | High | Moderate | Low |
|---|---|---|---|
| Prospective pattern | Prospective | Prospective | Prospective or retrospective |
| Ascertainment of exposure | Study began inside 1 yr of fluoridation | Study began within iii years of fluoridation | Report began >3 years after fluoridation |
| Ascertainment of outcome | Follow-up for at to the lowest degree 5 years and blind assessment | Long follow-upward and blind assessment | Short follow-up and unblinded assessment |
| Control for confounding | Adjustment for at least three confounding factors (or use of randomization) | Aligning for at least ane misreckoning factor | No adjustment for confounding factors |
STEP 4: SUMMARIZING THE Prove
To summarize the evidence from studies of variable design and quality is not like shooting fish in a barrel. The original review3 provides details of how the differences between study results were investigated and how they were summarized (with or without meta-analysis). This paper restricts itself to summarizing the findings narratively. The association betwixt exposure to fluoridated water and cancer in full general was examined in 26 studies. Of these, 10 examined all-crusade cancer incidence or mortality, in 22 analyses. Of these, 11 analyses found a negative association (fewer cancers due to exposure), 9 found a positive ane and 2 found no association. But 2 studies reported statistically meaning differences. Thus no clear association between h2o fluoridation and increased cancer incidence or bloodshed was credible. Bone/joint and thyroid cancers were of particular concern considering of fluoride uptake past these organs. Neither the half-dozen studies of osteosarcoma nor the ii studies of thyroid cancer and h2o fluoridation revealed significant differences. Overall no association was detected between water fluoridation and bloodshed from any cancer. These findings were likewise borne out in the moderate-quality subgroup of studies.
STEP 5: INTERPRETING THE FINDINGS
In the fluoridation instance, the focus was on the safety of a customs-based public health intervention. The generally low quality of available studies means that the results must exist interpreted with caution. Notwithstanding, the elaborate efforts in searching an unusually large number of databases provide some safeguard against missing relevant studies. Thus the testify summarized in this review is probable to be as practiced as it will get in the foreseeable hereafter. Cancer was the harmful outcome of almost involvement in this case. No clan was plant between exposure to fluoridated water and specific cancers or all cancers. The interpretation of the results may exist generally express considering of the low quality of studies, but the findings for the cancer outcomes are supported by the moderate-quality studies.
RESOLUTION
Afterward having spent some time reading and understanding the review, you lot are impressed past the sheer corporeality of published work relevant to the question of safety. However, you are somewhat disappointed by the poor quality of the primary studies. Of course, examination of safety only makes sense in a context where the intervention has some beneficial effect. Benefit and harm have to be compared to provide the ground for determination making. On the consequence of the beneficial result of public water fluoridation, the review3 reassures you that the wellness authority was correct in judging that fluoridation of drinking water prevents caries. From the review you besides discovered that dental fluorosis (mottled teeth) was related to concentration of fluoride. When the involvement groups raise the issue of rubber again, you will be able to declare that there is no testify to link cancer with drinking-water fluoridation; however, you will have to come up make clean almost the take a chance of dental fluorosis, which appears to be dose dependent, and y'all may desire to measure the fluoride concentration in the water supply and share this information with the interest groups.
The power to quantify the safety concerns of your population through a review, albeit from studies of moderate to low quality, allows your health dominance, the politicians and the public to consider the balance between beneficial and harmful furnishings of water fluoridation. Those who run across the prevention of caries as of primary importance will favour fluoridation. Others, worried about the disfigurement of mottled teeth, may prefer other means of fluoride assistants or even occasional handling for dental caries. Whatever the opinions on this matter, yous are able to reassure all parties that there is no testify that fluoridation of drinking water increases the gamble of cancer.
Decision
With increasing focus on generating guidance and recommendations for exercise through systematic reviews, healthcare professionals need to understand the principles of preparing such reviews. Here we have provided a brief step-by-footstep explanation of the principles. Our book1 describes them in particular.
References
Articles from Journal of the Royal Society of Medicine are provided here courtesy of Royal Social club of Medicine Printing
gilmoreyouresser1972.blogspot.com
Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC539417/
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